how to allocate memory to a member at run time

[raghu]

Hello ,

This is Raghu. I have a problem in declaring a structure.
Consider
struct hai{
int id;
char sex;
int age;
};
here when a variable is instianted for this structure then immediately
for all members memory is allocated. But I need to allocate memory for
age only if sex is M else the memory should not allocate.
Think you understand the query.

awaiting for your reply
bye take care
with smile
Raghu

 

[mark_bluemel]

Hello ,

This is Raghu. I have a problem in declaring a structure.

yes, you said - why ask twice?

 

[thunder1]

struct hai
{
int id;
char info[1];
}
If sex is M then p = (struct hai*)malloc(sizeof(struct hai)+1).
p->info[0] value is M,and p->info[1] is age.
If sex is F then p= (struct hai*)malloc(sizeof(struct hai)).p->info[0]
value is F.
raghu 写é“：

 

[Christopher Benson-Manica]

struct hai
{
int id;
char info[1];
}

If sex is M then p = (struct hai*)malloc(sizeof(struct hai)+1).
p->info[0] value is M,and p->info[1] is age.
If sex is F then p= (struct hai*)malloc(sizeof(struct hai)).p->info[0]
value is F.

1. Don't cast the return value of malloc(). Search this group's
archives and the FAQ.

2. OP stated that age is an int, not a char:

The struct hack is really not particularly helpful in OP's situation
anyway.

 

[Andrew Poelstra]

Hello ,

This is Raghu. I have a problem in declaring a structure.
Consider
struct hai{
int id;
char sex;
int age;
};
here when a variable is instianted for this structure then immediately
for all members memory is allocated. But I need to allocate memory for
age only if sex is M else the memory should not allocate.
Think you understand the query.

struct hai
{
int id;
char info[1];
}
If sex is M then p = (struct hai*)malloc(sizeof(struct hai)+1).
p->info[0] value is M,and p->info[1] is age.
If sex is F then p= (struct hai*)malloc(sizeof(struct hai)).p->info[0]
value is F.

Firstly, snip signatures and don't top-post.
Secondly, write your malloc()'s more robustly:

p = malloc(sizeof *p + 1);
p = malloc(sizeof *p);

 

[CBFalconer]

.... snip ...

Firstly, snip signatures and don't top-post.
Secondly, write your malloc()'s more robustly:

p = malloc(sizeof *p + 1);
p = malloc(sizeof *p);

Clearer is:

p = malloc(1 + sizeof *p)

which makes it absolutely clear that sizeof operates on *p alone.